This invention relates to optical fibers and a method of fabricating the same. More particularly, it concerns optical fibers having optical characteristics capable of providing polarization retaining or polarization locking properties and a method of fabricating the same.
In optical fibers, such as single-mode optical fibers, it is desirable that the optical fiber or optical waveguide have polarization locking or retaining properties by which the polarized state of the propagated light energy is retained throughout the complete length of the fiber. The polarization locking properties are utilized in single-mode optical waveguide communications as well as in physical condition sensors in which changes in a sensed condition cause measurable perturbations in the polarization of propagated light.
Prior single-mode optical waveguide fibers have utilized non-circular cores in an effort to achieve the desired polarization locking or retaining characteristics. In addition to the use of non-circular cores, optical fiber structures have been fabricated to place the core under loading forces on preestablished lateral or diametric axes to cause stress birefringence by fabricating the core and cladding components from glasses having different thermal coefficients of expansion (TCE). When light is propagated along fibers of this type, the propagated light energy polarizes on an axis generally parallel to the axis of the force-induced stress and on an axis perpendicular to the first axis due to slightly different stress-induced indices of refraction on the two orthogonal diametric axes.
While the prior techniques for achieving polarization locking or retaining properties in fiber optical waveguides have shown promise, the need for waveguide structures by which birefringence is physically induced represents a compromise as compared with a waveguide having inherent birefringence to display two different indices of refraction on mutually orthogonal diametric axes. Hence, there is a need for single-mode optical waveguides in which the material of the waveguide exhibits an inherent or pre-established birefringence.